Helical springs that are curved when unstressed are known from German Patents 1 505 616 and 3 743 450, for example. Springs of this type are employed to advantage in automotive-axle dashpots because they can be positioned with their curvature extensively compensating for undesirable friction and bending moments against the shock-absorber piston rod.
Curved helical springs have until now been manufactured only on cold-winding mechanism without mandrels as described for instance on pages 60 to 62 of Meissner and Wanke's "Handbuch Federn," Berlin, VEB Verlag Technik, 1988. This procedure achieves the desired curvature without the expense of varying every other winding flight.
Helical springs can in one particular alternative to winding without using mandrels be manufactured hot on what are called winding benches. They can be shaped by two parallel cylindrical rollers, one called the mandrel and the other the trainer. The training function can on the other hand be assumed by a computerized numerically controlled wire feed as described on pages 185 and 186 of the Hoesch Hohenlimburg AG literature "Warmgeformte Federn " issued on the occasion of the 52nd IAA in Frankfurt-am-Main in 1987 and on pages 106 and 107 of the Stahlwerke Bruninghaus GmbH literature "Technische Data Fahrzeugfedern" issued on the occasion of the 45th IAA in Frankfurt-am-Main in 1973.
A two-step method of manufacturing curved helical springs with existing production facilities (in-house state of the art) has been developed.
The first step comprises coiling a straight-cylindrical helical spring. The second step is to position it on a curved mandrel in a hardening press. The press is closed, curving the spring in conformity with the mandrel, and the spring is quenched in oil.
A substantial drawback of this two-stage method as compared to the method of manufacturing a straight spring is that it is more complicated and accordingly more expensive.
A method of manufacturing curved helical springs is known from German 4 124 326 C1. The curvature is obtained by incomplete cooling or heating of sections of the coil before or during setting.
Reproducible industrial-scale manufacture of curved helical springs by this known method extensively depends on local flow boundaries, which are directly determined by the spring's temper strength and local temperatures. It is accordingly necessary to achieve the desired curvature in addition to locally determined flow boundaries to apply tension during the setting process that precisely matches the particular cross-section or coil section. This would seem possible with today's demanding tolerances only with economically unjustified expenditures.